1. Field of the Invention
The present invention relates to a method and system for controlling a motor.
2. Background Art
Electric motors may be operated in any one of a number of different operating modesxe2x80x94e.g., torque control and speed control. The optimum mode of operation may depend on many factors, including the application in which the motor is being used, and the particular motor requirements when the motor is being run. One difference between a torque control mode and a speed control mode is the desired output of the motor. For example, when operating in a torque control mode, a motor controller may send a signal or signals to the motor to control the torque to achieve a desired motor torque. Conversely, when a motor is being operated in a speed control mode, the goal is a desired motor speed. Thus, even though the motor controller may still control the torque of the motor, the torque is controlled to achieve a desired motor speed.
Electric motors have many different applications, including use in electric vehicles and hybrid electric vehicles (HEVs). In electric vehicles and HEVs, the ride quality is directly related to the electric drive system""s ability to produce a smooth, continuous torque. For these vehicles to perform just as their xe2x80x9cengine onlyxe2x80x9d counterparts would, the electric motor must be operated in several different modes. These different modes will typically complicate the system control and thus require additional effort to offset their complexities. In HEVs, a vehicle system controller (VSC) is usually present to manage system functions and to interface with a traction drive system. A traction drive system may include the electric motor, a motor controller, and power electronics. The motor controller, which may include a speed controller, controls the motor through the power electronics, which manipulates magnetic fields within the motor to control the motor torque.
At a basic level, the electric motor is operated in the torque control mode; however, in many instances, the best vehicle performance requires that the motor be operated in the speed control mode. Transitioning the motor from one mode of operation to another can cause problems in powertrain performance. For example, as a motor is transitioned from torque control to speed control, the vehicle system controller provides a speed command, or target speed, to the motor controller. The target speed is then compared to the present motor speed, the difference between the two being a speed error. If this target speed is at or below the present motor speed, the speed error is zero or negative, and this momentarily sets the motor""s output torque to zero (or even a negative torque) as soon as the motor is transitioned into speed control. This situation is transient however, and as the motor decelerates, the speed error becomes positive and the torque of the motor begins to increase. This recovery takes time, and even if the system responds quickly to increase the torque after the motor decelerates, the initial drop in torque may cause the vehicle to experience a brief, sudden deceleration. In fact, an undesired xe2x80x9ctorque holexe2x80x9d may occur each time the operating mode is changed from torque control to speed control.
An example of where an electric motor in a vehicle may change from torque control to speed control is seen in the operation of an HEV. If the HEV is operating solely under the power of the electric motor, and the electric motor is operating in the torque control mode, the need to start the internal combustion engine may necessitate transitioning the motor from torque control mode to speed control mode. Thus, while the speed controller initializes, the potential for a torque hole would exist. Another example of where a motor in a motor driven vehicle may need to be transitioned from torque control to speed control is during the acceleration of the vehicle when a gear change is required. The gear change may force a rapid transition into speed control mode to aid in a smooth shift. This frequent switching from one mode to another is required for optimal vehicle performance, but also means that special attention must be paid to the transitions.
Accordingly, it is desirable to provide a method and system for transitioning an electric motor between two operating modes, such that the transitions may occur as frequently as needed to optimize vehicle performance, while eliminating undesired degradation in powertrain performance caused by a sudden drop in motor torque.
Therefore, a method of transitioning an electric motor from a first operating mode to a second operating mode is provided. The method comprises measuring the speed of the motor and generating a second torque command. The second torque command may be a function of the measured motor speed, a first torque command, and a motor speed command. The second torque command is sent to the motor, and it is modified when the measured motor speed reaches a predetermined value.
The invention further provides a method of transitioning an electric motor from a first operating mode to a second operating mode. The method comprises measuring the speed of the motor and generating a speed error. The speed error is the difference between a motor speed command and the measured motor speed. A second torque command is also generated. The second torque command may be a function of the speed error, an integral of the speed error, and a first torque command. The second torque command is sent to the motor, and it is modified when the measured motor speed reaches a predetermined value.
The invention also provides a system for transitioning an electric motor from a first operating mode to a second operating mode. The system comprises a first controller for at least controlling the motor. The first controller is configured to at least receive a measured motor speed, receive a first torque command, and receive a motor speed command. The first controller is also configured to generate a second torque command, the second motor torque being a function of the measured motor speed, the first motor torque, and the motor speed command. The first controller is also configured to output a second torque command.
The invention further provides a controller for controlling an electric motor. The controller comprises an algorithm for generating a second torque command and for sending the second torque command to the motor. The second torque command may be a function of a measured motor speed, a motor speed command, and a first torque command.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.